A honeycomb structural body consisting of ceramics having excellent heat resistance and corrosion resistance is used in various industrial fields such as chemical, electric power, steel, industrial waste processing or the like as several kinds of separator (filter, for example) from the view point of environmental protection measurements, such as prevention of air pollution, prevention of global warming or the like. A porous honeycomb structural body is suitably used as a dust collection filter such as a diesel. particulate filter (DPF), which is used under high temperature and corrosive gas atmosphere for capturing the particulates exhausted from the diesel engines, for example.
Recently, a porous honeycomb structural body having low pressure loss and high porosity is especially requested among the above mentioned porous honeycomb structural body from necessity of improving the capacity of filter for dust collection. As the method for producing such a high porosity honeycomb structural body, for example, the method of producing the porous honeycomb structural body, which comprises kneading main constituent material, water, an organic binder (it is necessary to use an organic binder such as methyl cellulose or the like for improving plasticity, because the smooth extrusion to honeycomb shape is difficult if only the forming material is used because of insufficiency of plasticity and formability), a pore forming agent (an organic substance such as graphite or the like) or the like as a forming material, preparing a green body of improved plasticity, extruding shaping and drying the green body to manufacture a honeycomb formed body, and firing the honeycomb formed body to obtain a porous honeycomb structural body, is disclosed (for example, see Patent document 1). By using this kind of manufacturing method, it is possible to obtain a porous honeycomb structural body having high porosity, because pores are formed by destruction (burning out) of the organic binder and the pore forming agents at the time of firing of the honeycomb formed body.
Patent Document 1: JP-2002-219319
However, there is a problem which may cause to reduce the mechanical strength of the ultimate porous honeycomb structural body, because the obtained calcined body may have defects which were the space occupied by the organic binder at the time of calcining (binder removal) if the amount of addition of the organic binder is too much. Furthermore, the thermal stress between inner and outer part of the shaped body, which is produced by burning the organic binder at the time of calcining (binder removal) because of the inner part of the shaped body may have higher temperature than the outer part of the shaped body, may cause defects such as cracks or the like in the calcined body, especially if the bulk of the structural body is large.
The present invention has been made in view of the above problems and aims to provide a method for producing a porous honeycomb structural body, which method enables to obtain a calcined body having less defects such as cracks or the like and high mechanical strength after calcining (binder removal), and also enables ultimately to obtain a porous honeycomb structural body having high porosity and high quality, and also provide to the porous honeycomb structural body having high porosity and high quality obtained by this method.